1. Field of the Invention
This invention relates to a method of preparing a durable air-permeable mold having fine and continuous pores all over it and having a fine pattern on its surface.
2. Prior Art
Such a durable air-permeable mold can be, for example, used for the vacuum-molding method. This method is one in which after a thermoplastic sheet is heated and softened, it is made to cover a mold, the air-permeable surface of which is subjected to suction, and then the air present between the surface of the mold and the thermoplastic sheet is withdrawn to cause the sheet to adhere to the surface of the mold.
Since this method has merits in that small to large molded products can be efficiently produced with relatively simple equipment, it has been widely utilized. Recently, fine patterns such as are obtained by embossing the surface of molded products are in demand to enhance the commercial value of molded products, or molds to obtain large molded products and high-dimensional accuracy are demanded.
Also, such durable porous molds can be widely used not only for vacuum-molding, but also for blow-molding and other methods that require air permeability. As molds that satisfy such requirements, as shown in Japanese Patent Publication No. 2-6620, the applicant has developed molds excellent in durability as well as having low costs, short-term delivery, and with good transferability and air-permeability.
Additionally, it has developed various air-permeable and water-permeable molds that are disclosed in Japanese Patent Publication Nos. 63-65416, 1-53140, 2-27925, 3-23253, etc.
All of these molds are obtained by mixing a liquid binder with a mixture of metal powders and ceramic powders or refractory powders, their specific gravities being very different, to form a slurry, casting the slurry into a pattern mold, solidifying the binder, drying the thus-obtained solid product, and then sintering the product in an oxidative atmosphere to oxidize and expand the metal powders to obtain an air-permeable mold with high strength, being sintered in a form such that the oxidized and expanded metal powders envelope the ceramic powders.
However, these molds have a drawback in that in the above slurry a mixture of the ingredients of the aggregate starts to precipitate before the binder solidifies, and when the binder is solidified the segregation of the ingredients occurs since a difference between the sedimentation rate of the metal powders and that of the ceramic powders is apt to be caused, said metal powders and ceramic powders significantly differing in their specific gravities.
For this reason the molds obtained by sintering the solidified product with the segregation produce cracks and a surface irregularity due to the segregation. Since these molds have very good transferability, such cracks and surface irregularity are transferred to a molded product. Thus it is difficult to obtain molds which can produce good molded products. Also, since the metal powders provide the strength, to obtain a high-strength mold the amount of metal powders tends to be high (for example, the amount of metal powders against the amount of ceramic powders can be 1:1). Accordingly, sintered molds tend to expand, and it is difficult to control dimensional accuracy.